FIG. 1 shows a typical optical lithographic system 200 for delineating features in a wafer (substrate) 100 or in one or more layers of material(s) (not shown) located on a top major surface of the wafer, typically a semiconductor wafer (substrate). More specifically, optical radiation from an optical source 106, such as a mercury lamp, propagates through an aperture in an opaque screen 105, an optical collimating lens 104, a mask or reticle 103, and an optical focusing lens 102. The optical radiation emanating from the reticle 103 is focused by the lens 102 onto a photoresist layer 101 located on the top major surface of the wafer 100 itself or, alternatively, on the layer(s) on the top surface of the wafer 100. Thus, the pattern of the reticle 103--that is, its pattern of transparent and opaque portions--is focused on the photoresist layer 101. Depending upon whether this photoresist is positive or negative, when it is subjected to a developing process, typically a wet developer, the material of the photoresist is removed or remains at and only at areas where the optical radiation was incident. Thus, the pattern of the mask is transferred to ("printed on" ) the photoresist layer 101. Subsequent etching processes, such as wet etching or dry plasma etching, remove selected portions of the substrate or of the layer(s) of material(s) (not shown) located between the top major surface of the wafer and the bottom surface of the photoresist layer, or of both the substrate and the layer(s). Portions of the substrate or of the layer(s) of material thus are removed from the top surface of the wafer 100 at areas underlying where the photoresist layer 101 was removed by the developing process but not at areas underlying where the photoresist remains. Thus, the pattern of the mask 103 is transferred to the wafer 100 or to the layer(s) of material(s) overlying the wafer 100, as is desired, for example, in the art of semiconductor integrated circuit fabrication.
In fabricating such circuits, it is desirable to have as many devices, such as transistors, per wafer. Hence it is desirable to have as small a transistor or other feature size as possible, such as the feature size of a metallization stripe--i.e., its width W--or of an aperture in an insulating layer which is to be filled with metal, in order to form electrical connections, for example, between one level of metallization another. Thus, for example, if it is desired to print the corresponding isolated feature having a width equal to W on the photoresist layer 101, a feature having a width equal to C must be located on the mask (reticle) 103. According to geometric optics, if this feature of width equal to C is a simple aperture in an opaque layer, then the ratio W/C=m, where m=L2/L1, and where m is known as the lateral magnification. When diffraction effects become important, however, the edges of the image become fuzzy (lose their sharpness); hence the resolution of the mask features when focused on the photoresist layer deteriorates.
In a paper entitled "Exploration of Fabrication Techniques for Phase-Shifting Masks" published in Proc. SPIE. (The Society of Photo-Optical Instrumentation Engineers)--The International Society for Optical Engineering--Optical/Laser Microlithography IV, Vol. 1463, pp. 124-134 (March, 1991), A. K. Pfau et al. taught the use of masks having transparent phase-shifting portions in an effort to achieve improved resolution--i.e., improved sharpness of the image of the mask features focused on the photoresist layer 101. More specifically, these masks contained suitably patterned transparent optical phase-shifting layers, i.e., layers having edges located at predetermined distances from the edges of the opaque portions of the mask. Each of these phase-shifting layers had a thickness t equal to .lambda./2(n-1), where .lambda. is the wavelength of the optical radiation from the source 106 (FIG. 1) and n is the refractive index of the phase-shifting layers. Thus, as known in the art, these layers introduced phase shifts (delays) of .pi. radians in the optical radiation. By virtue of diffraction principles, the presence of these phase-shifting layers should produce the desired improved resolution. Such masks are called "phase-shifting" masks.
The mask structure described by A. K. Pfau et al., op. cit., was manufactured by a single-alignment-level process involving forming a chromium layer on a top major surface of a quartz substrate, followed by patterning the chromium layer into segments by means of lithographic masking and etching. Next, a positive resist layer was formed over the resulting structure. The resist layer was bombarded with a patterned beam of actinic radiation having edges located somewhere on every other chromium segment. Then a dry plasma anisotropic etching is performed, using the combination of the patterned resist layer and the patterned chrome layer as a protective mask against the etching, whereby the desired phase-shifting mask structure was obtained (after removal of the patterned resist layer). No (precise) alignments were thus required. However, in such a case, when the dry plasma etching is performed, the then exposed edge regions of the chrome layer may undesirably be removed or chemically changed, whereby the reflectivity of the resulting chrome layer is not desirably uniform across the surface of the chrome; so that, when the resulting lithographic mask is used in an optical lithography system, undesirable optical noise can result. Also, in such a case, during the dry etching of the substrate the width of remaining patterned chrome layer undesirably can be changed, whereby line-width control is undesirably lost; and moreover (nonvolatile) chromium fluoride can deposit on the (quartz) substrate, whereby its optical transmission is undesirably reduced when the resulting phase-shifting mask (of which the substrate is a part) is used for optical lithography.
Therefore, it would be desirable to have a method of manufacturing phase-shifting masks that requires no (precise) alignment level and that ameliorates the shortcomings of prior art.